1. Field of the Invention
The present invention relates to fuel cell separator compositions and to fuel cell separators made therewith as well as a method of manufacturing such fuel cell separators. The invention also relates to a solid polymer fuel cell assembled using such fuel cell separators as some or all of the separators therein.
2. Prior Art
Fuel cells are devices which convert chemical energy directly into electrical energy by placing a pair of electrodes in mutual contact through an intervening electrolyte, feeding a fuel to one of the electrodes and an oxidant to the other electrode, and carrying out oxidation of the fuel electrochemically within the cell. There are several types of fuel cells, depending on the electrolyte used. Solid polymer fuel cells in which the electrolyte is a solid polymer electrolyte membrane have attracted considerable attention recently for their ability to achieve a high energy output.
As shown in FIG. 1, such solid polymer fuel cells are composed of a stack of from several tens to several hundreds of unit cells, each unit cell having a pair of fuel cell separators 1, 1 with a plurality of ribs la on either side thereof, between which separators 1 are disposed a solid polymer electrolyte membrane,2 and a pair of gas diffusing electrodes (a fuel electrode and an oxidant electrode) 3, 3.
In the illustrated solid polymer fuel cell, a stream of hydrogen is supplied to the fuel electrode, a stream of oxygen is supplied to the oxidant electrode, and the electrical current produced by the cell is drawn off by an external circuit. The reactions which take place at the respective electrodes can be represented as follows.
Fuel electrode reaction: H2xe2x86x922H++2exe2x88x92 (1)
Oxidant electrode reaction: 2H++2exe2x88x92+xc2xdO2xe2x86x92H2O (2)
Overall reaction: H2+xc2xdO2xe2x86x92H2O
That is, hydrogen (H2) is converted into protons (H+) at the fuel electrode. The protons then migrate through the solid polymer electrolyte membrane to the oxidant electrode, where they react with oxygen (O2) to form water (H2O). The supply and removal of reactant and product gases and the drawing off of electrical current are thus essential to operation of the solid polymer fuel cell. Moreover, it is presumed that the solid polymer fuel cell will generally be operated in a wet environment within a temperature range of room temperature to 120xc2x0 C., meaning that water will be handled in a liquid state. Arrangements must therefore be made to control the supply of water to the fuel electrode and to remove water from the oxidant electrode.
Of the components making up this type of fuel cell, the fuel cell separator, as shown in FIGS. 2A and 2B, has the distinctive shape of a thin plate provided on one or both sides thereof with a plurality of flow channels 4 for the supply and removal of gases. It plays several important roles, one of which is to separate the fuel gas, oxidant gas, and cooling water flowing through the fuel cell to keep them from mingling. In addition, it transmits from the fuel cell electrical energy generated within the cell, and dissipates out of the fuel cell heat that forms within the cell. Accordingly, a need has been strongly felt for fuel cell separators which, in addition to gas barrier properties, electrical conductivity and corrosion resistance, also have sufficient mechanical strength to resist cracking and breaking of the separators from the tightening of bolts and nuts during fuel cell assembly, and which moreover are endowed with excellent vibration and impact resistance when the fuel cell is used as a mobile power supply for automotive and other similar applications.
Carbon composites in which various thermoplastic or thermoset resins offering certain advantages in terms of ease of production and cost are employed as binders have already been proposed for use in solid polymer fuel cell separators of this type. Examples include carbon composites in which the following are used as the binder: the phenolic and other thermoset resins described in JP-A 59-26907; and the polypropylene, nylon and other thermoplastic resins described in JP-A 56-116277.
However, fuel cell separators made of carbon composites in which such thermoplastic or thermoset resins are used as the binder, while preferable from the standpoint of production and cost to the separators machined from graphite sheets that were previously used, leave much to be desired in terms of such performance characteristics as mechanical strength, chemical resistance, gas permeability, and dimensional stability. An additional drawback is the need for degassing during the separator molding or forming operation.
It is therefore one object of the invention to provide a fuel cell separator composition from which there can be obtained a fuel cell separator that is capable of being mass-produced and has excellent electrical conductivity, mechanical strength, chemical resistance, gas impermeability, low ion extraction, and excellent molding or forming properties. Another object of the invention is to provide a fuel cell separator made from the same fuel cell separator composition. A further object is to provide a method of manufacturing such a fuel cell separator. A still further object is to provide a solid polymer fuel cell which has been assembled using such fuel cell separators as some or all of the separators therein.
We have discovered that fuel cell separator compositions wherein a mixture of a thermoset resin with a polyoxazine compound having a plurality of oxazine rings is employed as the binder for an electrically conductive carbonaceous powder can be molded or formed into fuel cell separators which have improved high-temperature durability, unlike the prior art wherein use of a binder composed of a thermoplastic or thermoset resin alone results in an inadequate high-temperature durability, and which are also endowed with better mechanical strength, chemical resistance, gas barrier properties and water resistance, lower ion extraction, better dimensional stability, and little or no need for degassing during molding or forming.
We have also found that solid polymer fuel cells assembled using such separators of excellent electrical conductivity, mechanical strength, chemical resistance, gas impermeability, low ion extraction, and molding or forming properties as some or all of the separators within the fuel cell undergo minimal decline in energy output and have a high operating efficiency, even when the separator has high gas barrier properties and the fuel cell is continuously operated for a long period of time. Hence, such fuel cells are well suited for use as mobile electrical power supplies for such applications as conventional automobiles, hybrid cars and small boats.
Accordingly, in a first aspect, the invention provides a fuel cell separator composition which includes an electrically conductive carbonaceous powder and a binder, wherein the binder is a mixture of a thermoset resin with a polyoxazine compound having a plurality of oxazine rings. The composition typically contains 100 parts by weight of the thermoset resin and 5 to 200 parts by weight of the polyoxazine compound. Preferably, the composition includes 100 to 6,000 parts by weight of the conductive carbonaceous powder per 100 parts by weight of the thermoset resin, and the conductive carbonaceous powder has an average particle size of 10 nm to 500 xcexcm. It is advantageous for the composition to include also up to 500 parts by weight of a fibrous base per 100 parts by weight of the thermoset resin.
In a second aspect, the invention provides a fuel cell separator made by imparting to a fuel cell separator composition according to the above-described first aspect of the invention a separator shape having gas supplying and removing channels oh one or both sides thereof. The separator, when a 3.5 g specimen is cut therefrom, placed in 305 ml of deionized water and heated at 90xc2x0 C. for 500 hours, imparts to the water an electrical conductivity of not more than 20 xcexcS/cm. It is preferable for the fuel cell separator to have a resistivity, as measured according to JIS H0602, of at most 50 mxcexa9xc2x7cm and a gas transmission rate, as measured by method B of JIS K7126, of at most 50 ml/m2 xc2x7dayxc2x7atm.
In a third aspect, the invention provides a method of manufacturing fuel cell separators, which method includes the steps of preparing a fuel cell separator composition comprising an electrically conductive carbonaceous powder and a binder which is a mixture of a thermoset resin with a polyoxazine compound, then shaping the composition into a fuel cell separator having gas supplying and removing channels on one or both sides thereof. The composition is prepared by the admixture of 100 parts by weight of the thermoset resin, 5 to 200 parts by weight of the polyoxazine compound, 100 to 6,000 parts by weight of the conductive carbonaceous powder, and 0 to 500 parts by weight of a fibrous base.
In a fourth aspect, the invention provides a solid polymer fuel cell made up of a plurality of stacked unit cells, each unit cell being composed of a solid polymer electrolyte membrane, a pair of electrodes disposed on either side of the polymer electrolyte membrane, and a pair of separators disposed on either side of the pair of electrodes such as to form gas supplying and removing channels. At least some of the separators within the fuel cell are fuel cell separators according to the above-described second aspect of the invention. Preferably, the solid polymer fuel cell has an initial voltage V1 and a voltage V2 after 200 to 500 hours of continuous operation, such that (V2/V1)xc3x97100 is at least 80%.